Multimode antenna system

ABSTRACT

A waveguide structure, designed to operate as a wide-band multimode antenna, has an elongate main waveguide section centered on an axis and terminating at a radiating aperture, an intermediate waveguide section of lesser width joined to that main section, and a pair of still narrower input sections feeding the intermediate section, these input sections being cophasally or antiphasally excited with the same fundamental mode TE10. A first discontinuity, at a transverse plane representing the junction between the input and intermediate waveguide sections, and a second discontinuity, at a transverse plane representing the junction between the intermediate and main waveguide sections, give rise to mixed TEM modes which come into existence at the first or the second discontinuity, depending upon wavelength; at the originating discontinuity, the mixed mode is in phase with the generating fundamental mode so that, with proper dimensioning of the length of the main waveguide, they will recombine cophasally at the radiating aperture.

0 n, r tt tates Patent 1 [111 3,75,t3 Drahewitch fly 31, R973 MULTHMODEANTENNA SYSTEM [75] Inventor: Serge Drabowitch, Paris, France ABSTRACT[73] Assi e; Thom o -(13F, Pa i France A waveguide structure, designedto operate as a wideband multimode antenna, has an elongate main wave-[22] 1971 guide section centered on an axis and terminating at a [21] ApL No.1 208,060 radiating aperture, an intermediate waveguide section oflesser width joined to that main section, and a pair of still narrowerinput sections feeding the intermediate [30] Foreign ApplicationPriority Data section, these input sections being cophasally or anti-Dec. 22, [970 France 7046249 phagaliy excited with the same fundamentalmode TEW A first discontinuity, at a transverse plane representing ELS-(Hi. the junction between the input and intermediate wave. [5 flint. Cl.guide sections and a econd digeoni inuily at 3 "ans. [58] Field ofSearch /771, 333/6 verse plane representing the junction between theintermediate and main waveguide sections, give rise to References (medmixed TEM modes which come into existence at the UNITED STATES PATENTSfirst or the second discontinuity, depending upon wave- 3,573,838 4/1971Ajioka 343/786 i f at the P s s dissfmtinuity, the mixed mode 3,373,4313/1968 Webb IS in phase with the generating fundamental mode so3,530,483 9/1970 Pierrot..... that, with proper dimensioning of thelength of the 3,566,309 2/1971 Ajioka 343/786 main waveguide, they willrecombine cophasally at the Primary Examiner-451i LiebermanAttorney-Karl F. Ross radiating aperture.

10 Claims, d Drawing Figures PATENIEU 1 3. 750. 1 83 SHEET 1 BF 2 SergeDRABOWITCH Inventor 5 By A g ()0 Attorney PATENTEB I975 3. 750, 1 83 sum2 BF 2 WAVE- ENERGY INPUT Serge DRABOWITCH Inventor By WA g'iRossAttorney 1 MULTIMODE ANTENNA SYSTEM The present invention relates toimprovements in multimode antennas.

Multimode antennas are radiating devices which are based uponsimultaneous radiation of different guided propagation modes which aregenerated by devices known as moders and whose amplitudes and phases arecontrolled. Under these conditions, the the resulting radiant-energydistribution or beam pattern is determined by the superimposition of thefundamental and harmonic guided-propagation modes. The creation of thesemodes and their superimposition have been achieved in structures, morespecifically known as moders," which are constituted by waveguides andembody discontinuities designed to generate higher modes from thefundamental mode injected at the input ot the structure. My prior U.S.Pat. No. 3,308,469 discloses moders of this kind in which one or morewaveguides known as excitation waveguides, each filtering itsfundamental propagation mode, terminate in the input of a main waveguidewhich has a definite length and'whose open end in principle constitutesthe radiating opening. This main waveguide is designed to propagatevarious higher modes and the harmonic modes are generated in principleat the planes of the discontinuities at which the excitation waveguidesterminate. FIG. 1 of the accompanying drawing illustrates such a moderwhich can now be considered as conventional. The main waveguide l, oflength L, communicates with two excitation waveguides 2 and 3, thelatter terminating in the main waveguide at the plane 2 known as thediscontinuity plane. The excitation waveguides 2 and 3 are supplied withthe fundamental mode whereas harmonic modes are generated at thisdiscontinuity plane; the harmonic modes, together with the fundamentalmode, are propagated through the principal waveguide 1, whose dimensionsare chosen accordingly, up to the opening or aperture plane Z L wherethey are radiated. Naturally, the moder of FIG. 1 has been illustratedpurely schematically and may be supplemented with, for example, modefilters for eliminating unwanted modes, matching devices associated withthe excitation waveguides, and also other discontinuities intentionallyintroduced in order to modify the ratio of the generated modes.

However, the fundamental and harmonic modes generated in accordance withthe foregoing are propagated through the main waveguide at differentphase velocities up to the radiating aperture at location 2 L. Theresult is that the precise phase combination required at the radiatingaperture is strictly achieved only at a single frequency. However, ifthe modes utilized are not too close to cut-off it can be shown thatvery stable energy distribution can be obtained through frequency bandshaving a width on the order of 10 percent.

in certain applications, and in particular in space telecommunications,it is necessary to provide a passband whose width is on the order of 12to 14 percent; a bandwidth on the order of 18 percent is alsocontemplated.

The object of the present invention, is to provide, in a moder of theclass described in the aforementioned patent, a novel structure whichoperates over a substantially wider passband than has been obtained withselective moders of prior-art design.

An antenna system according to my present invention comprises awaveguide strucutre with at least two stepped junctions, ordiscontinuities, between an excitation stage, including a plurality ofinput waveguide sections, an intermediate waveguide section, and a mainwaveguide section terminating in a radiating aperture. The intermediatewaveguide section has a crosssection which is smaller than that of themain waveguide section but larger than that of each input waveguidesection and, in fact, of the several input waveguide sections combined.These input waveguide sections are simultaneously excited, with a commonfundamental mode of propagation, from a source of wave energy at afrequency which is transmissible by the structure.

The main waveguide section may be centered on an axis and may have acircular cross-section, with the stepped junctions lying in planestransverse to that axis. The input and intermediate waveguide sections,in the embodiment described hereinafter, are of rectangularcross-section similar to corresponding waveguides in the antenna systemof my prior U.S. Pat. No. 3,308,469.

As will be described more fully hereinafter, the length of the mainwaveguide section should be calculated to produce substantial in-phaserelationship at the radiating aperture between the energy propagating atthe fundamental mode from the input stage and energy propagating at ahigher mode, generated at the last junction, for a minimum transmissiblefrequency giving rise to this higher mode.

The above and other features of my invention will now be described indetail with reference to the accompanying drawing in which:

FIG. 1, already referred to, is a perspective view of a prior-art moderaccording to my U.S. Pat. No. 3,308,469;

FIG. 2 is a similar schematic view of a moder in accordance with theinvention;

FIG. 3 is a schematic axial sectional view of a simplified type-E moderembodying my invention; and

FIG. 4 is a similar view of another embodiment of the invention.

In accordance with the invention, the widening of the passband of amoder depends upon the phase velocity of the modes propagating withinits structure, and upon its length which is determined in such a fashionthat the modes are in phase in the plane of the radiating aperture.

FIG. 2 schematically illustrates my improved waveguide strucutre in wichthe main waveguide 1 is pre ceded by a conventional moder of the kinddisclosed in my above-identified patent. in this Figure, the compositeunit 4 is a flat E-type moder whose excitation or input sections 5 and 6are both supplied in the fundamental mode (TE cophasally or in antiphasedepending upon the parity of the modes which it is desired to generatein an intermediate section waveguide 8,'the dimensions of the lattersection having been chosen accordingly. Another horn-type waveguide 9 isshown as well, with its discontinuity plane 10. These multimodeexcitation assemblies terminate in the main waveguide 1 whose inputplane 11 constitutes a discontinuity plane or stepped junction. In thisprincipal waveguide, the modes generated at the discontinuity planessuch as 7 and 11 combine in phase and amplitude to produce the resultantin-phase modes at the radiating aperture.

The novel structure thus has two axially spaced discontinuities locatedin the junction planes between the several differently sized waveguidesections.

FIG. 3 illustrates another multimode structure embodying my inventionwhich has the characteristics set out hereinbefore and is simplified forthe sake of easier explanation of its operation. This schematic viewillustrates a longitudinal section of an E-type moder but theexplanation which follows applies equally to an H-type moder also comingwithin the scope of the invention.

This moder structure in accordance with the invention comprisesexcitation stage 5, 6 between the input plane 12 and the plane of afirst discontinuity 13. Between the plane 13 and the plane 14 of asecond discontinuity, there is an intermediate waveguide section 15.Beyond the plane M, the main waveguide section 16 extends up to theplane 17 of the radiating aperture of the system.

The excitation of input sections and 6 are assumed to be supplied withthe fundamental TE mode and the two successive discontinuities l3 and 14create the TE and TM modes which have the same phase velocity. These twomodes combine to produce the mixed mode TEM g.

Excited in phase at the discontinuity planes, these TE and TEM modespropagate at their own phase velocities and if they are to be in phaseat the end of the main waveguide through which they propagate, then thelength L of the latter must satisfy the condition where A and A are thewavelengths of the two modes in question.

Unter these circumstance, and in accordance with the dimensions of theintermediate and main waveguides 15, 16 it is observed that at the lowfrequencies of the range the higher mixed mode TEM is cut off in theintermediate waveguide and does not start to propagate until beyond thesecond discontinuity 14. The system then operates like a conventionalmoder with only one discontinuity. At the high frequencies of the range,the higher mixed mode is generated at the first discontinuity 13,propagates through the intermediate waveguide and thus excites the mainwaveguide 16. At the second discontinuity 14 a function of the highermode TEM is generated which combines in amplitude phase with the modegenerated by the first discontinuity.

The two mode functions which combine at the second discontinuity are outof phase and, in accordance with equation (I), the length of the mainwaveguide is such that this equation is satisfied. Under theseconditions, the wave components will attain a cophasal relationship atthe plane of the radiating aperture where, for this length of waveguide,the desired distribution is achieved. In fact, in accordance with thefrequency at which the system is operated, equation (I) is satisfied bythe introduction ofan equivalent length L, as if the resultant TEM modewere generated in a plane located between the two discontinuity planesl3 and 14. Thus, when the frequency is relatively low, the plane ofgeneration of the resultant mode is that of the second discontinuity,but as the operating frequency increases, this plane of generation ofthe higher mode tends to move further away from the plane of the seconddiscontinuity l4 and to move towards the plane of the firstdiscontinuity 13. The equivalent length L of the structure in accordancewith the invention thus increases with the frequency, the physicallength ot the main waveguide being chosen for the lowest frequency inthe range.

Under these conditions, a bandwidth on the order of 20 percent has beenobserved, with phase errors at the radiating end of the multimodewaveguide not in excess of 6.

FIG. 4 illustrates a further multimode antenna structure whichconstitutes a generalization of the embodiment hereinbefore described.In this flat E-typc moder structure the two discontinuity planes 13 and14 are still present but this time there terminate at the plane of thesecond discontinuity i. e., at the entrance end of main waveguidesection 16, two supplementary excitation waveguides 18, 19 designed togenerate odd modes, e.g. TEM This kind of arrangement makes it possibleto carry out range measurements in a monopulse system by the explorationof difference channels. The operation of this kind of system is similarto that already described. At the low frequencies in the band inquestion, the higher mode disappears at the first discontinuity 13 andis only propagated beyond the level of the second discontinuity 14. Asthe frequency rises, the system behaves as if the resultant modeproduced in the principal waveguide had been generated at a plane movingfrom the second discontinuity 14 to the first in the taperingintermediate waveguide 20 of progressively increasing cross-section anda function of the higher mode is generated at the plane of the firstdiscontinuity 13.

In this system, beyond the second discontinuity and in the principalwaveguide 21, I provide shield means to produce theoretically perfectdecoupling between the supplementary waveguides 18, 19 and the compositewaveguide 22 for the excitation of the even modes. This shield meansconsists of metal plates 23 24 disposed transversely to the electricfield of the fundamental mode.

In FIGS. 3 and 4 the intermediate waveguide section 15 or 20 is shown tobe coaxial with main waveguide section 16 whereas the input waveguidesections 5, 6 or .22a, 22b are symmetrically disposed with reference totheir axis. Sections 22a, 22b of FIG. 4 are seen to open into a furtherwaveguide section 220 ahead of section 20, with formation of anadditional stepped junction or discontinuity at 22d.

Thus, a novel wideband multimode structure has been described whichpresents at least two successive, stepped discontinuities. It should beborne in mind in this context that the height of the steps is not amatter of arbitrary choice. It depends upon the proportion of the highermodes which have to be generated in order to produce the radiationpattern required at the plane of the radiating aperture of the system.

Similarly, it is evident that the number of discontinuities used is afunction ofthe width of the desired passband. The number of cascadeddiscontinuities is in no way limitative and may be increased withoutdeparting from the principles of the invention.

It is likewise possible, without departing from the scope of the presentinvention, to provide discontinuities in a flat H-type moder and tocombine a flat E-type moder and H-type moder.

Of course, the invention is not limited to the embodiments described andshown which are given solely by way of example.

What is claimed, is:

1. An antenna system comprising a waveguide structure with a mainwaveguide section terminating at a radiating aperture, an intermediatewaveguide section coupled to said main waveguide section at an endthereof remote from said aperture, said intermediate waveguide sectionhaving a cross-section smaller than that of said main waveguide section,and a plurality of input waveguide sections of still smallercross-section coupled to said intermediate waveguide section at an endthereof remote from said main waveguide section, said input andintermediate waveguide sections forming a stepped first junction, saidintermediate and main waveguide sections forming a stepped secondjunction; and a source of wave energy of a frequency transmissible bysaid waveguide structure connected to said input waveguide sections forsimultaneously exciting same with a common fundamental mode ofpropagation.

2. A system as defined in claim 2 wherein said main waveguide section iscentered on an axis, said first and second junctions lying in planestransverse to said axis.

3. A system as defined in claim 2 wherein said input and intermediatewaveguide sections extend parallel to said axis.

4. A system as defined in claim 2 wherein said input and intermediatewaveguide sections are of rectangular cross-section, said main waveguidesection being of circular cross-section. I

5. A system as defined in claim 2 wherein said input waveguide sectionsare a pair symmetrically positioned with reference to said axis.

6. A system as defined in claim 1 wherein the combined cross-section ofsaid input waveguide sections is less than the cross-section of saidintermediate waveguide section.

7. A system as defined in claim 1 wherein said intermediate waveguidesection progressively increases in cross-section from said first to saidsecond junction.

8. A system as defined in claim 1, further comprising supplemental inputmeans entering said intermediate waveguide section at said firstjunction.

9. A system as defined in claim 8, further comprising conductive shieldmeans in said intermediate waveguide section adjacent to said firstjunction for mutually decoupling said supplemental input means and saidinput waveguide sections.

10. A system as defined in claim 1 wherein said main waveguide sectionhas a length calculated to produce substantial in-phase relationship atsaid radiating aperture between energy propagating at said fundamentalmode and energy propagating at a higher mode, generated at said secondjunction, for minimum transmissible frequencies giving rise to saidhigher mode.

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1. An antenna system comprising a waveguide structure with a mainwaveguide section terminating at a radiating aperture, an intermediatewaveguide section coupled to said main waveguide section at an endthereof remote from said aperture, said intermediate waveguide sectionhaving a cross-section smaller than that of said main waveguide section,and a plurality of input waveguide sections of still smallercross-section coupled to said intermediate waveguide section at an endthereof remote from said main waveguide section, said input andintermediate waveguide sections forming a stepped first junction, saidintermediate and main waveguide sections forming a stepped secondjunction; and a source of wave energy of a frequency transmissible bysaid waveguide structure connected to said input waveguide sections forsimultaneously exciting same with a common fundamental mode ofpropagation.
 2. A system as defined in claim 2 wherein said mainwaveguide section is centered on an axis, said first and secondjunctions lying in planes transverse to said axis.
 3. A system asdefined in claim 2 wherein said input and intermediate waveguidesections extend parallel to said axis.
 4. A system as defined in claim 2wherein said input and intermediate waveguide sections are ofrectangular cross-section, said main waveguide section being of circularcross-section.
 5. A system as defined in claim 2 wherein said inputwaveguide sections are a pair symmetrically positioned with reference tosaid axis.
 6. A system as defined in claim 1 wherein the combinedcross-section of said input waveguide sections is less than thecross-section of said intermediate waveguide section.
 7. A system asdefined in claim 1 wherein said intermediate waveguide sectionprogressively increases in cross-section from said first to said secondjunction.
 8. A system as defined in claim 1, further comprisingsupplemental input means entering said intermediate waveguide section atsaid first junction.
 9. A system as defined in claim 8, furthercomprising conductive shield means in said intermediate waveguidesection adjacent to said first junction for mutually decoupling saidsupplemental input means and said input waveguide sections.
 10. A systemas defined in claim 1 wherein said main waveguide section has a lengthcalculated to produce substantial in-phase relationship at saidradiating aperture between energy propagating at said fundamental modeand energy propagating at a higher mode, generated at said secondjunction, for minimum transmissible frequencies giving rise to saidhigher mode.